Geoscience Reference
In-Depth Information
a collective term for the group of partially
halogenated organic species, and includes the
chlorofluorocarbons (CFCs). Other smaller
factors with a positive radiation forcing include
tropospheric ozone, black carbon on snow
(essentially soot from fossil fuel burning) and
solar irradiance (which is of course not associated
with human activities). These positive forcings
are in part countered by negative forcings due to
increased aerosol concentration and increased
surface albedo associated with land use, yielding
the estimated total forcing due to human activities
of about 1.6W m
-2
. The uncertainty in this value
is largely due to uncertainty in the aerosol effects.
Because of their highly episodic nature,
Figure
13.4
does not include the effects of volcanic
eruptions.
While CFCs (one of the halogens) have a
positive radiative forcing, the student may be
more familiar with the link between CFCs and
the destruction of stratospheric ozone. Despite
the Montreal Protocol that has helped to control
the production and use of CFCs, CFCs are long-
lived and are still impacting upon the ozone layer
(see Chapter 2A.4). Emissions of H
2
O and NOx by
jet aircraft and by surface emissions of N
2
O are
contributing to the problem. Ozone circulates in
the stratosphere from low to high latitudes and
thus the occurrence of ozone in polar regions is
diagnostic of its global concentration. In October
1984, an area of marked ozone depletion (termed
the 'ozone hole') was observed in the lower
stratosphere (i.e., 12-24km) centered on, but
extending far beyond, the Antarctic continent.
Ozone depletion is always greatest in the Antarctic
spring, but in that year the ozone concentration
was more than 40 percent lower than in October
1977. By 1990, Antarctic ozone concentra-
tions had fallen to about 200 Dobson units in
September to October (see
Figure 2.9
), compared
with 400 units in the 1970s. In the extreme years
(1993-1995), record minima of 116 DU have
been recorded at the South Pole. It has been
estimated that, owing to the slowness of the global
circulation of CFCs and of their reaction with
ozone, even a cut in CFC emissions to the level of
that in 1970 would not eliminate the Antarctic
ozone hole for at least 50 years. Winter ozone
depletion also occurs in the Arctic stratosphere
and was well marked in 1996 and 1997, but absent
in 1998. Localized mini-holes are fairly common,
but extensive holes are rare even in cold
stratospheric winters. It seems that whereas the
Antarctic vortex is isolated from the mid-latitude
circulation, the Arctic vortex is more dynamic
so that transport of ozone from lower latitudes
makes up much of the loss.
Aerosol forcings are both direct and indirect.
Together, they have an estimated radiative forcing
of about -1.2W m
-2
. The direct effects relate to
how aerosols absorb and scatter both solar and
longwave radiation; a variety of aerosol types,
including fossil fuel organic carbons, fossil fuel
black carbon, biomass burning and mineral dust
and sulfate aerosols, exert a significant radiative
forcing. The indirect effect relates to how aerosols
alter clouds. A key issue is how effectively an
aerosol particle can act as a cloud condensation
nucleus, which depends on factors such as the
chemical composition and size of the aerosol. The
indirect aerosol effect includes impacts on cloud
albedo (often termed the first indirect effect) and
impacts on cloud liquid water, height and lifetime
(the second indirect effect). Reducing the high
uncertainty in the direct and indirect effects of
aerosols is a key focus area of climate research.
Regarding land use, the basic issue is that
increasing population pressures have led to over-
grazing and forest clearance, acting to increase
the planet's surface albedo. While the radiative
forcing relative to 1750 is a modest - 0.2Wm
-2
,
human effects on vegetation cover have a long
history. Burning of vegetation by Aborigines in
Australia has been traced over the last 50,000
years, while significant deforestation began in
Eurasia during Neolithic times (about 5000 years
ago), as evidenced by the appearance of agricul-
tural species and weeds. Deforestation expanded
